1. Field of the Invention
The present invention relates to a method of producing a through-hole in a silicon wafer, a substrate used to produce a through-hole, a device using such a substrate, a method of producing an ink-jet print head, and an ink-jet print head.
2. Description of the Related Art
In recent years, research and development has been carried out in the art of micromechanics with the objective of realizing a micromachine having an ultrasmall movable mechanism. In particular, the technique of forming a microstructure on a single-crystal silicon substrate using semiconductor integrated circuit technology (semiconductor photolithographic process) is promising in that a plurality of ultrasmall mechanical elements can be produced on the substrate with high reproducibility. This technique allows a plurality of such ultrasmall mechanical elements in an array form at a reduced cost. Furthermore, the reduction in the size of elements can result in a high response speed compared with the conventional mechanical structure. In the art of the micromechanics based on the semiconductor photolithographic process, bulk micro-machining is an essentially important technique to produce a high-precision through-hole used to realize a thin-film cantilever or nozzle. The bulk micro-machining technique is based on the technique of etching a silicon substrate by means of a crystal orientation-dependent anisotropic etching process in which etching for (111) crystal surfaces occurs at a different rate from that for other crystal surfaces. The technique of producing a through-hole by etching a silicon substrate from its back side by means of a crystal orientation-dependent anisotropic etching process is useful to produce various devices such as a cantilever and a micro valve on the surface of the substrate, and therefore intensive research and development is being carried out with the objective of realizing various devices using this technique.
One known device using a cantilever is a cantilever probe used in a scanning probe microscope (hereinafter also referred to as an SPM). The advent of scanning tunnel microscope s capable of directly observing electron structures of atoms on the surface of a conductor (G. Binnig et al., Phys. Rev. Lett., 49, 57(1983)) has made it possible to obtain a high-resolution microscopic spatial image of an object regardless of whether the object is in a single crystal form or an amorphous form. Thus, the SPM is now widely used to evaluate the microstructure of specimens. To improve the performance and function of the SPM, thin-film cantilevers having various capabilities realized in an integral fashion have been proposed. For example, in the atomic force microscope capable of measuring the microscopic structure of the surface of a specimen by means of detecting repulsive and attractive force at the surface of a substance, it has been proposed to use a piezoresistance cantilever having a piezoresistance integrated on a cantilever instead of a conventional cantilever using an optical lever to detect deflection (M. Tortonese et al., "Atomic Force Microscopy using a Piezoresistive Cantilever", The 6th International Conference on Solid-State Sensors and Actuators, Transducers '91, 1991, pp. 448-451). Using such a piezoresistance cantilever, it is possible to detect a microscopic surface structure even in vacuum or at a low temperature without needing external detection devices such as a laser, an optical component, and a photodetector.
A method of producing such a piezoresistance cantilever by means of silicon crystal orientation-dependent anisotropic etching is described below with reference to FIG. 20.
First, an SOI wafer 500 serving as a substrate is prepared by forming a silicon dioxide layer 502 and an n-type silicon layer 503 on a p-type silicon substrate 501 (refer to FIG. 20A). A silicon dioxide layer 504 is then formed on the principal surface and also the back surface of the SOI wafer. The silicon dioxide film 504 on the principal surface is removed, and boron (B) is implanted and diffused into the n-type silicon layer 503 thereby forming a resistor pattern 505 in the shape of a cantilever in the n-type silicon layer. Furthermore, a thin silicon dioxide film 507 serving as a passivation layer is formed on the cantilever and a contact hole is then formed therein. Subsequently, an aluminum metal electrode 508 is formed thereon. An opening 506 for use as an etching window is formed in the silicon dioxide film 504 on the back surface of the SOI wafer (FIG. 20B). The p-type silicon substrate is then etched via the opening 506 by EDP (ethylenediamine/pyrocatechol) serving as a crystal orientation-dependent anisotropic etchant for silicon thereby forming a hole surrounded by (111) surfaces of the silicon substrate and the membrane of the silicon dioxide layer 502. The silicon dioxide layer 502 is partially removed using hydrofluoric acid thereby forming a through-hole thus forming a piezoresistance cantilever (FIG. 20C).
In the above technique of producing a through-hole by etching a silicon substrate from its back surface using a crystal orientation-dependent anisotropic etchant, the opening length d at the principal surface of the substrate is, as shown in FIG. 21, determined by the opening length D at the back surface of the substrate, the substrate thickness t, and the crystal orientation-dependent anisotropic etchant employed. When a (100) silicon substrate is used, the opening length d is approximately given by EQU d.about.(D-2t/tan(54.7.degree.)+2Rt/sin(54.7.degree.)) (1)
where R is the ratio of the etching rate for the (111) surface to that for the (100) surface. Thus it is possible to obtain cantilever having a desired length simply by controlling the opening length D to a proper value depending on the material of the cantilever and the thickness of the substrate. Therefore, it is possible to produce a cantilever having a desired resonance frequency and a spring constant. In a similar manner, it is also possible to produce a nozzle having a desired orifice diameter. As described above, various devices having a cantilever or a nozzle on the surface of a substrate can be produced by etching a silicon substrate from its back side using a crystal orientation-dependent anisotropic etchant thereby forming a through-hole. In both examples described above, the length of the cantilever and the diameter of the orifice are determined by the opening length.
However, silicon wafers vary in thickness and orientation flat indicating the crystal axis, from wafer to wafer and from lot to lot, due to variations in production conditions. For example, the wafer-to-wafer and lot-to-lot variations of 4 inch diameter silicon wafers are 500 .mu.m to 525 .mu.m in thickness (thickness variation .DELTA.t=25 .mu.m) and .+-.0.4.degree. in crystal axis. Thus, when a (100) wafer with a diameter of 4 inches is used, a variation .DELTA.d of about 35 .mu.m occurs in the opening length of the through-hole measured at the surface of the substrate due to the thickness variation .DELTA.t, from wafer to wafer or from lot to lot.
Because the back side opening is patterned with respect to the orientation flat, the variation in the orientation flat angle results in a variation in the angle of the back side opening. Therefore, in the case where there is a variation in the orientation flat angle of the order described above, if a through-hole having an opening length of 1000 .mu.m measured at the surface is produced, a variation of about 12 .mu.m can occur in the opening length from wafer to wafer or from lot to lot.
As described above, when the through-hole is formed by etching the silicon substrate from its back surface, a variation .DELTA.d occurs in the opening length due to the variations in production parameters such as substrate thickness and orientation flat angle. As a result, when a cantilever is produced, the opening length variation .DELTA.d causes a variation of order of a few ten .mu.m in the length of the cantilever. Therefore, the mechanical characteristics such as resonance frequency and spring constant of the produced cantilever vary from substrate to substrate. This makes it difficult to produce a cantilever without encountering wafer-to-wafer variations in the mechanical characteristics.
KOH and EDP used as a crystal orientation-dependent anisotropic etchant are highly toxic and difficult to deal with. To avoid the problem of toxicity, TMAH (tetramethyl ammoniumhydoroxide) has come to be used recently instead of KOH or EDP. TMAH is low in toxicity and contains no metal ions, and thus it is an excellent etchant having good compatibility with LSI processes. TMAH has a property that the ratio R of the etching rate for a (100) surface of silicon to that for a (111) surface varies with the concentration of TMAH (U. Schnakenberg et al., "TMAHW Etchants for Silicon Micromachining", The 6th International Conference on Solid-State Sensors and Actuators, Transducers 91, 1991, pp. 815-818). For example, when the concentration of TMAH is 22 wt %, the etching rate ratio R will be 0.03, while the etching rate ratio R will be 0.05 when the concentration of TMAH is 10 wt %. If such a variation in the etching rate ratio R is taken into account in equation (1), it can be seen that an opening length variation .DELTA.d of 27 .mu.m occurs owing to the variation in the TMAH concentration when the substrate thickness is maintained at 525 .mu.m. This means that when a through-hole is produced using TMAH, the variation in the opening length d is affected not only by the variations in the substrate thickness and the orientation flat angle but also by the variation in the concentration of the etchant, and thus the variation in the opening length becomes greater.
One known technique of producing a nozzle having a desired opening length on a silicon substrate is to form a high-concentration p-type diffusion layer on the silicon substrate (E. Bassous, "Fabrication of Novel Three-Dimensional Microstructures by the Anisotropic Etching of (100) and (110) Silicon", IEEE Trans. on Electron Devices, Vol. ED-25, No. 10, 1978, p. 1178-). This technique utilizes the property that a p-type diffusion layer with an impurity concentration higher than 7.times.10.sup.19 cm.sup.-3 is not etched by a crystal orientation-dependent anisotropic etchant. In this technique, an orifice is formed as follows. First, a silicon dioxide film is formed on a silicon substrate. The silicon dioxide film is then patterned into the shape of an orifice. Boron (B) is diffused into the substrate to a high impurity level thereby forming a p-type diffusion layer. Another silicon dioxide layer is then formed thereon, and an opening is formed in the silicon dioxide film on the back surface of the substrate. Subsequently, the silicon substrate is etched by a crystal orientation-dependent anisotropic etchant thereby forming a nozzle surrounded by (111) surfaces of the silicon substrate and a membrane of p-type diffusion layer having an orifice. Although this technique is capable of producing a high-precision orifice, this technique has a problem that the thickness of the membrane is as small as 3 .mu.m. To increase the thickness of the membrane, high-concentration ion implantation is required, and thus a long time is required to perform ion implantation. Furthermore, a long diffusion time is required to achieve a thicker diffusion layer. For example, to obtain a diffusion layer with a thickness of 15 to 20 .mu.m, impurity ions should be implanted to a level of as high as 1.times.10.sup.16 or higher atoms/cm.sup.2. Furthermore, it is also required to perform diffusion at 1175.degree. C. for a time as long as 15 to 20 hours. This results in a reduction in productivity. If a silicon substrate is subjected to high-temperature treatment for a long time, crystal defects can occur in the bulk of silicon crystal or the defect density increases. The crystal defects can cause anomalous etching to the (111) surfaces during the crystal orientation-dependent anisotropic etching process, thus causing deformation of the shape of the opening end from the ideally linear shape. As a result, a variation occurs in the opening length d measured at the surface of the substrate.
When an electronic circuit is integrated on a silicon substrate, it is required to perform a heat treatment at a temperature for a time similar to those described above so as to form an nMOS well and an insulating diffusion layer. The density of crystal defects generated during heat treatment varies across a wafer and varies from lot to lot. Such a variation in the defect density can cause a variation in the opening length d from opening to opening. When a micromechanical device and an electronic device are integrated together, crystal defects can cause deformation of the shape of the opening end at the surface of the substrate from the ideally linear shape. Another problem of this technique is that it is impossible to form a diffusion layer below a previously-formed device such as a piezoresistance cantilever on an SOI substrate.
In view of the above-described problems in the conventional techniques, the object of the present invention is to provide a method of producing a through-hole, a substrate used to produce a through-hole, a substrate having a through-hole, and a device using such a through-hole or a substrate having such a through-hole, which are characterized in that:
(1) A through-hole can be produced only by etching a silicon substrate from its back side; PA1 (2) The opening length d can be precisely controlled to a desired value regardless of the variation in the silicon wafer thickness from wafer to wafer or from lot to lot; PA1 (3) The opening length d can be precisely controlled to a desired value regardless of the variation in the orientation flat angle from wafer to wafer or from lot to lot; PA1 (4) The opening length d can be precisely controlled to a desired value regardless of the type of a silicon crystal orientation-dependent anisotropic etchant employed; PA1 (5) High productivity, high production reproducibility, and ease of production can be achieved; PA1 (6) A high-liberality can be achieved in the shape of the opening end even if temperature treatment is performed at a high temperature for a long time; and PA1 (7) A high-precision through-hole can be produced regardless of the shape of a device formed on the surface of a substrate.